A couple of thoughts:
Unlikely but she may have had a vanity plate.
He is an accountant, numbers are his game, maybe it comes easy to remember that many digits/letters?
I went out with someone once a very long time ago and he could tell you whose vehicle was whose to the tune of tons of people, almost identical vehicles he could differentiate, who was driving a vehicle when we passed someone and where almost everyone lived. Long story short, I came to find out that these were the actions of a cheater and someone who knows where all women live if they interested him, even the married ones, if their car was outside a business, etc., if their husband was at work, etc., etc.
They are likely also the actions of some stalkers and KS certainly could fit that in this case...
I know my mother's vehicle for sure when I see it in a parking lot because the three letters on the plate are the consonants in a word and Z is one of them so I could always determine if she was say at the grocery store when I happened to see her vehicle in the parking lot and was heading shopping myself. However, I could not tell you what the three numbers are so I do find it unusual he knew her plate number.
The strange thing is he stated this to someone, that he saw a plate like hers or some such... This hints to me that he does not think this is abnormal to know her or someone's plate number. I can tell you the person I am talking about above also did not realize what he did and did not realize knowing so much was fairly unusual so he showed what he knew without realizing it was a giveaway which over a short bit of time, I figured out the reason for.
Are we sure he said plate
number? If he said instead a plate that looked like hers, it could have been a scenic plate or some such from another state or something and he saw another one from that state one day... In my state, we can pay extra to get certain plates instead of a standard one.
To me, finally, it still makes I guess the most sense he was stalking her though and that is why he knew the plate number. It sounds as if he was lying in wait so he knew where she lived as well and how to get there, he probably had been before.
P.S. I want to be clear that I am not saying that everyone who knows people's plates or can remember them is odd lol, I just do not think it is that common amongst people and in this case, I think it shows she was on his radar and probably had been for some time.
All just speculation and jmo